The present invention relates to a particle control screen for depth filtration, particularly for use in a well.
Liquids and gases in oil and gas wells typically include particulates that need to be filtered, including sand, clay, and other unconsolidated particulate matter. The presence of sand and other fine particles in the production fluid and well equipment often leads to the rapid erosion of expensive well machinery and hardware.
Subterranean filters, also known as sand screens or well screens, have been used in the petroleum industry to remove particulates from production fluids. The well screens are generally tubular in shape and include a perforated base pipe, a porous filter layer wrapped around and secured to the pipe, and an outer cover. The well screens are used where fluid enters a production string, such that the production fluid must pass through the filter layer and into the perforated pipe prior to entering the production string and being pumped to the surface.
In the context of downhole filtration, woven wire mesh is considered surface filtration, which means that the mesh prevents particles of the desired micron size and larger from passing through the mesh and all the particles are trapped on the top surface of the mesh. Wire wrap is also a common type of surface filtration. Wire wrap is usually triangular shaped wire wrapped around a base pipe, with a given gap between wires to accomplish a micron rating. One difficulty with surface filtration is that as larger particles are captured on the filter layer, the open spaces become smaller and smaller, thus capturing smaller and smaller particles. Eventually the particles being captured are so fine that the filter becomes plugged, severely reducing or stopping flow of formation fluids through the screen to the base pipe.
Large reserves of thick, viscous hydrocarbons exist in locations such as the Orinoco belt in Venezuela and the oil sands of Alberta, as well as fields in Sumatra, China, Brazil, the North Sea, and Kazakhstan. Different names, such as heavy oil, extra heavy oil, oil sands, or bitumen are used to describe the material. Heavy oil is an asphaltic, dense (i.e. low API gravity), and viscous oil that is chemically characterized by the presence of asphaltenes, which are very large molecules incorporating most of the sulfur and metals in the oil. Heavy oil generally has a gravity of less than 22 degrees API gravity and a viscosity of greater than 100 centipoise. Extra-heavy oil is heavy oil having an API gravity of less than 10 degrees. Natural bitumen, also called tar sands or oil sands, generally has a viscosity greater than 10,000 centipoise. Oil sands can include as low as 10% bitumen and 85% or more clay, sand, and rocks. Heavy oil is more difficult to remove from the formation and also includes more particulate matter than conventional oil deposits. Thus, heavy oil is generally also harder to filter than conventional oil deposits.
Thus, there is a need for a downhole filter assembly with improved filtering performance, and especially for use with heavy oil.